1. Field of The Invention
The present invention relates to a light receiving member sensitive to electromagnetic waves such as light (light in a broad sence such as UV-rays, visible rays, infrared rays, X-rays and y-rays).
2. Related Background Art
For photoconductive materials to constitute a light receiving layer of a light receiving member for use in the image-forming field, it is required that they have high sensitivity, high S/N ratio (photocurrent (IP)/dark current (ID)), absorption spectrum characteristics suited to electromagnetic waves to be irradiated, rapid responsibility to light and desired dark resistance, as well as they are not harmful to human bodies. In particular, for light receiving members to be employed in electrophotographic apparatus which are used as business machines at the office, it is important that they cause no public pollution during use.
In recent years, photoconductive materials comprising amorphous silicon (a-Si) materials such as amorphous silicon hydrides (hereinafter referred to as a-Si:H) have been evaluated to satisfy these requirements. Particularly, there are a number of proposals for the use of such amorphous silicon photoconductive material in an electrophotographic light receiving member.
For instance, U.S. Pat. No. 4,265,991 discloses an electrophotographic light receiving member in which such a-Si photoconductive material is used.
In the production of a light receiving member comprising such a-Si material, there is generally employed a manner of forming an a-Si deposited film as a photoconductive layer on a substrate maintained at a temperature of 50 to 350.degree. C. by a film-forming process by means of vacuum evaporation, sputtering, ion plating, thermal-induced CVD, light-induced CVD, or plasma CVD.
Of these film-forming processes, the film-forming process by means of plasma CVD in which glow discharge is caused in a silicon-containing raw material gas to decompose said raw material gas whereby forming an a-Si deposited film on a substrate maintained at a desired temperature has been evaluated as being preferable and it has been frequently employed in the production of a light receiving member.
U.S. Pat. No. 5,382,487 discloses an electrophotographic light receiving member comprising an electrically conductive substrate and a photoconductive layer constituted by an a-Si material containing halogen atoms (X) (this a-Si material will be hereinafter referred to as a-Si:X). This document describes that the incorporation of halogen atoms in an amount of 1 to 40 atomic % enables to form a photoconductive layer having high heat resistance and good electrical and optical properties which is suitable for use in an electrophotographic light receiving member.
Japanese patent application Laid-open No. 57-115556 discloses that for a photoconductive member having a photoconductive layer comprising an a-Si deposited film, in order to attain improvements in its electrical, optical and photoconductive properties such as dark resistance, photosensitivity, and photoresponsibility, use environmental characteristics such as moisture resistance, and stability with time, a surface barrier layer constituted by a non-photoconductive amorphous material containing silicon and carbon atoms is disposed on a photoconductive layer constituted by an amorphous material containing silicon atoms as a matrix.
Japanese patent application Laid-open No. 60-67951 discloses a photosensitive member having a light transmissive insulating overcoat layer containing amorphous silicon, carbon, oxygen and fluorine.
U.S. Pat. No. 4,788,120 discloses that an amorphous material containing silicon atoms, carbon atoms and hydrogen atoms in an amount of 41 to 70 atomic % is used as a surface layer in a light receiving member.
Japanese patent application Laid-open No. 62-83470 discloses that for a photoconductive layer of an electrophotographic photosensitive member, it is made to have less than 0.09 eV of a characteristic energy of the exponential tail in a light absorption spectrum, whereby attaining a high quality image with on ghost.
Japanese patent application Laid-open No. 58-21257 discloses a technique of obtaining a photosensitive member having a high electric resistance and a widened photosensitive region by changing the substrate temperature during the formation of of a photoconductive layer therefor to make the resulting photoconductive layer have a varied forbidden band width.
Japanese patent application Laid-open No. 58-121042 discloses a technique of varying the energy gap state density of a photoconductive layer in the layer thickness direction to make a surface layer have an energy gap state density of 10.sup.17 to 10.sup.19 cm.sup.-3, whereby preventing the surface potential from being decreased due to moisture.
Japanese patent applications Laid-open Nos. 59-143379 and 61-201481 disclose a technique of obtaining a photosensitive member having a high dark resistance and a high photosensitivity by stacking a plurality of a-Si:H films each having a different hydrogen content.
Japanese patent application Laid-open No. 60-95551 discloses a technique of improving the quality of an image reproduced by an a-Si photosensitive member by conducting image-formation comprising charging, exposure, development and transferring processes while maintaining the temperature in the vicinity of the surface of the photosensitive member at 30 to 40.degree. C. thereby to prevent occurrence a smeared image along with a reduction in the surface resistance of the photosensitive member which is caused due to moisture absorbed at the surface of the photosensitive member.
According to the techniques described in the above documents, it is possible to attain electrical, optical and photoconductive characteristics, and use-environmental characteristics at a certain level for a light receiving member for use in electrography, whereby an improvement can be attained in the quality of an image reproduced by the light receiving member. However, for a light receiving member having a photoconductive layer constituted by an a-Si material for use in electrophotography, there still exists a room for a further improvement in view of overall characteristics including electrical, optical and photoconductive characteristics such as dark resistance, photosensitivity, and photoresponsibility, use-environmental characteristics, stability and durability upon continuous use over a long period of time.
In addition, in recent years, there has been made a rapid progress in electrophotographic apparatus such that they have a further improved image reproduction performance so as to reproduce a higher quality image and further improvements in their reproduction speed and durability. In this connection, for a light receiving member used in such electrophotographic apparatus, there is an increased demand for still further improving the electrical and photoconductive properties so as to exhibit a significantly upgraded image-forming performance in any environment while attaining a still further improvement not only in the charge retentivity but also in the photosensitivity.
Particularly, for the electrophotographic apparatus, as a result of having made improvements on their optical exposure devices, developing devices, transfer devices and so forth so so to reproduce a higher quality image as above described, there is such increased demand as above described for the light receiving member used therein.
Recent years, electric computers have been widely using in not only offices but also general houses and digitalization of character images have been progressing. Along with this, there is an increased demand also for digitalizing not only a conventional electrophotographic apparatus but also an electrophotographic apparatus which will be developed in the future since these electrophotographic apparatus pay a role as a facsimile or printer. For semiconductor lasers or LEDs (light-emitting diodes) used for the digitalization, there are usually used those having a long wavelength of infrared region to red visible light region. In this case, some problems concerning the characteristics of the light receiving member used, which can be disregarded in the case of an analogue electrophotographic apparatus, become serious and are, therefore, necessary to be solved.
These problems are possible to be solved at a certain level by the foregoing techniques in the prior art. However, any of the conventional light receiving members is not satisfactory in such digital electrophotographic system and is necessary to be more improved particularly with respect to the charge retentivity and image-forming performance so as to stably and continuously reproduce a high quality image also in the digital system.
For instance, in order for a conventional amorphous silicon series light receiving member to attain reproduction of a high quality in the digital electrophotographic system, it is required to minimize the variation in the electrophotographic characteristics due to a change in the environmental temperature, light fatigue, blank memory, and exposure memory such as ghost. And in the digital electrophotographic system using a laser or LED having a long wavelength as above described, it has become necessary to have a due care about temperature-dependent sensitivity in which a gradient of a curve of the interrelations between the quantity of light irradiated and the charge retentivity of a light receiving member is changed due to a change in the environmental temperature and sensitivity linearity where said gradient is dulled into a non-linear state, of which no attention has been made in the past.
In fact, in the prior art, in order to prevent a conventional light receiving member from causing a smeared image on an image reproduced, as described in Japanese patent application Laid-open No. 60-95551, a drum heater is installed in an electrophotographic apparatus to maintain the surface of the light receiving member at about 40.degree. C.
However, in the case of the conventional light receiving member, the temperature dependency of the charge retentiviy (or so-called temperature-dependent property in other words) due to generation of a pre-exposure carrier or thermally excited carrier is so great that there is no way other than to use the light receiving member in a state with a lower charge retentivity than that originally possessed by the light receiving member under the practical use environment in the electrophotographic apparatus. For instance, the charge retentivity of the light receiving member is liable to drop by about 100 V when heated to about 40.degree. C. by the drum heater, in comparison with the case when used at room temperature.
Incidentally, at night when an electrophotographic apparatus (or an electrophotographic copying machine) is not used, there is sometimes taken a measure of keeping the drum heater installed therein electrified in order to prevent an ozone product caused by corona discharge of the charger from depositing on the surface of the light receiving member, which results in causing a smeared image on an image reproduced. However, it tends to refrain taking such measure for the purposes of saving natural resources and electric-power. In this case where the drum heater of the electrophotographic apparatus is kept electrified without being suspended, a problem is liable to entail in that when copying shot is continuously conducted, the surrounding temperature of the light receiving member in the electrophotographic apparatus is gradually increased to cause a raise in the temperature of the light receiving member, where the charge retentivity of the light receiving member is deteriorated to vary the density of an image reproduced.
In addition, in the case of a digital electrophotographic copying machine in which laser beam is used, when the light receiving member used therein is not maintained constant at a desired temperature by means of a temperature controller such as the foregoing drum heater, a problem is liable to entail in that the surrounding temperature of the light receiving member is changed to cause a variation in the sensitivity of the light receiving member due to the foregoing temperature-dependent sensitivity or sensitivity linearity, where the density of an image reproduced is varied.
Besides, when an original is repeatedly and continuously subjected to reproduction, a variation in the density of an image reproduced and occurrence of a fogged image are sometimes occurred because of light fatigue of the light receiving member due to repeated image exposure. In addition, blank memory and so-called ghost are problematic in the case of attaining an improvement in the quality of an image reproduced, said blank memory being a phenomenon which causes a density variation on images reproduced due to the effects of so-called blank exposure which is applied onto the light receiving member in order to save the amount of toner used, said ghost being a phenomenon in which an image remaining after the image exposure in previous reproduction is appeared on an image obtained in the subsequent reproduction.
Accordingly, in designing a light receiving member for use in electrophotography, it is required to achieve improvements from the overall viewpoints of layer and chemical composition of each constituent layer of the light receiving member so that the foregoing problems can be desirably solved, and also to achieve a much more improvement in the property of the a-Si materials themselves.